The present application relates to a light source device configured as a MOPA (master oscillator power amplifier) in which a mode-locked laser and a semiconductor optical amplifier are combined. It also relates to an optical pickup and recording device that use a MOPA as a light source for recording on an optical recording medium.
An example of a pulse laser that achieves a relatively high repetition frequency such as hundreds of megahertz or several gigahertz includes a MOPA (master oscillator power amplifier) that uses an MLLD (mode locked laser diode) as the master laser and amplifies and modulates its optical output with an SOA (semiconductor optical amplifier).
A structure in which input light from a master laser is amplified and modulated by an SOA was mainly adopted in the optical communication field.
In optical communication, light to be input or output is transferred through an optical fiber.
In this case, especially in a backbone communication system, use of single mode fibers is specified by communication standards of, for example, ITU (International Telecommunication Union). Accordingly, waveguides for an SOA are often designed for single mode on the incident side. For actual products, in a semiconductor optical amplifier (semiconductor optical module) in which a semiconductor and an optical fiber are integrally packaged, connection with a single mode fiber is made as in Alphion products. Accordingly, the waveguide of a semiconductor optical amplifier is designed for single mode.
Designing of a waveguide for single mode in response to an input by a single mode fiber is obvious from the mode coupling theory (see “Lightwave Optics” by Yasuo Kokubun, Kyoritsu Shuppan).|Ein>=ΣiCi|φi>+ΣjDj|φj>  (1)where Ein is incident electric field, Φi is the waveguide mode of the waveguide at the input end of the semiconductor, and φj is the emission mode of the waveguide at the input end of the semiconductor.Ci=<φi|Ein>  (2)Di=<φj|Ein>  (3)where i and j are mode numbers. In addition, the following expressions hold because of its orthogonality.<φi|φk>=δik  (4)<φi|φk>=δik  (5)<φi|φj>=0  (6)
In addition, the following expression holds.<a|b>=∫−∞∞dx∫−∞∞dy a(x,y)*b(x,y)  (7)
In recent years, the applicant has proposed optical recording systems that use a MOPA as a recording light source (see, for example, Applied Physics Express 3 (2010) 102501 “Volumetric Optical Recording Using a 400 nm All-Semiconductor Picosecond Laser” Shiori Tashiro, Yoshihiro Takemoto, Hisayuki Yamatsu, Takahiro Miura, Goro Fujita, Takashi Iwamura, Daisuke Ueda, Hiroshi Uchiyama, KyungSung Yun, Masaru Kuramoto, Takao Miyajima, Masao Ikeda, and Hiroyuki Yokoyama, Storage & Memory Business Development Division, Core Device Development Group, Sony).
The MOPA proposed in the above cited document “Volumetric Optical Recording Using a 400 nm All-Semiconductor Picosecond Laser” achieves a peak power of approximately 100 W at a wavelength of approximately 405 nm, a repetition frequency of approximately 1 GHz, and a pulse width of approximately 4 picoseconds.
For confirmation, the structure of a MOPA used in an optical recording system will be described with reference to FIG. 20.
In FIG. 20, the MOPA has an MLLD unit 100 and an SOA 107. The MLLD unit 100 includes an MLLD 101 as a semiconductor laser and an external resonator (including a focusing lens 102, a band pass filter 103, and a resonant mirror 104) and emits a pulsed laser light (master laser light) with a predetermined repetition frequency.
Exit light from the MLLD unit 100 is collimated by a collimation lens 105 and the collimated light is focused by the focusing lens 106 on the incident end (entrance) of the SOA 107. The laser light incident through the focusing lens 106 is amplified and modulated by the SOA 107 and then output. The master laser light from the MLLD unit 100 is modulated by this SOA 107 in response to data to be recorded and then output.
Also in the MOPA used in this type of optical recording system, the lateral mode (horizontal lateral mode) of the waveguide on an incident side on the SOA 107 is designed for single mode, as in the optical communication field. In other words, the waveguide of the related-art SOA 107 has a structure that guides only single mode, that is, basic mode.
Accordingly, in the related-art MOPA, the output waveguide of the MLLD 101 and the input waveguide of the SOA 107 were designed to have the same dimension (same width) for efficient single mode optical coupling.